1. Field of Invention
This invention relates generally to communications, and more particularly, X-DSL modems.
2. Description of the Related Art
North American Integrated Service Digital Network (ISDN) Standard, defined by the American National Standard Institute (ANSI), regulates the protocol of information transmissions over telephone lines. In particular, the ISDN standard regulates the rate at which information can be transmitted and in what format. ISDN allows full duplex digital transmission of two 64 kilo bit per second data channels. These data rates may easily be achieved over the trunk lines, which connect the telephone companies' central offices. The problem lies in passing these signals across the subscriber line between the central office and the business or residential user. These lines were originally constructed to handle voice traffic in the narrow band between 300 Hz to 3000 Hz at bandwidths equivalent to several kilo baud.
Digital Subscriber Lines (DSL) technology and improvements thereon including: G.Lite, ADSL, VDSL, SDSL, MDSL, RADSL, HDSL, etc. all of which are broadly identified as X-DSL have been developed to increase the effective bandwidth of existing subscriber line connections, without requiring the installation of new fiber optic cable. An X-DSL modem operates at frequencies higher than the voice band frequencies, thus an X-DSL modem may operate simultaneously with a voice band modem or a telephone conversation.
X-DSL modems are typically installed in pairs, with one of the modems installed in a home and the other in the telephone companies central office (CO) switching office servicing that home. This provides a direct dedicated connection to the home from a line card at the central office on which the modem is implemented through the subscriber line or local loop. Modems essentially have three hardware sections: (a) an analog front end (AFE) to convert the analog signals on the subscriber line into digital signals and convert digital signals for transmission on the subscriber line into analog signals, (b) digital signal processing (DSP) circuitry to convert the digital signals into an information bit stream and optionally provide error correction, echo cancellation, and line equalization, and (c) a host interface between the information bit stream and its source/destination. Typically all of these components are located on a highly integrated single line card with a dedicated connection between one or more AFE's and a DSP.
Within each X-DSL protocol there are at least two possible line codes, or modulation protocols; i.e. discrete multi-tone (DMT) and carrierless AM/PM (CAP). The first of these line codes, i.e. DMT, requires the DSP to implement both an inverse fast Fourier transform (IFFT) on upstream data received from the subscriber and a fast Fourier transform (FFT) on the downstream data transmitted to the subscriber. Typically the DSP is available as a discrete semiconductor chip which implements the transforms for a dedicated one of the X-DSL standards using software routines running on an internal processor.
Each X-DSL installation represents a sizeable expense in hardware and service labor to provision the central office. The expense may not always be amortized over a sufficient period of time due the relentless introduction of new and faster X-DSL standards each of which pushes the performance boundaries of the subscriber line in the direction of increasing bandwidth and signal integrity. As each new standard involves, line cards must typically be replaced to upgrade the service.
What is needed is a less rigid signal DSP processing architecture that allows a more flexible hardware response to the evolving X-DSL standards and the problems associated with providing hardware to handle each new standard.